The present invention relates generally to systems and methods for printing, reproducing or displaying images. More particularly, the teachings disclosed herein are applicable to methods and apparatuses wherein dispersed dot halftoning is implemented.
Color is a powerful tool and has become an essential component of communication. The use of color facilitates the exchange of knowledge and ideas. Color can sway thinking, influence perceptions, change actions, and cause reactions. Similarly, the use of images is integral to communication and can be applied to a range of applications including, for example, explaining concepts and ideas, marketing goods and services, prompting responses and inspiring new ideas. The ability to faithfully reproduce images and colors is essential to communication as inaccuracies in color or poor image quality can defeat the purpose of or entirely destroy the intended communication. Those involved in the development of document processing systems or devices such as marking engines, printers, scanners, fax machines, electronic libraries, and the like to construct, produce, print, transmit, scan, store and archive documents and their constituent elements are continuously looking for ways to improve the accuracy and total image quality of their products.
Digital images are commonly represented as one or more separations, with each separation conventionally represented as a monochromatic bitmap, which may be described as an electronic image with discrete signals (hereinafter, pixels) defined by position and density. Monochromatic images typically are represented with a single separation while color images and documents commonly are represented as two or more separations with each separation comprising a set of color density signals for a single primary or secondary color. In monochromatic and color applications, density is described as one level in a number of possible states or levels. When more than two levels of density are used in a separation, the levels are often termed “gray”, indicating that they vary between a maximum and minimum, and without reference to their actual color.
Common input devices including document scanners, digital cameras and computer imagery generators are capable of describing an image with a large number of gray levels, with 256 levels a commonly selected number, although larger and smaller levels are possible. In such systems, image density signals are commonly represented as continuous tone (contone) pixels, varying in magnitude from a minimum to a maximum, with a number of gradations between corresponding to the bit density of the system. Thus, a common 8-bit system provides 256 density levels or shades of color for each separation.
Within a stage of the printing process of many printing devices, the potential image signal gradations are reduced to a limited number of possibilities, and are commonly binary, e.g., they either produce a dot or not at a given location. This quantization resolution reduction is due to the physical processes involved are binary in nature or have been restricted to binary operation for reasons of cost, speed, memory or stability (e.g., ink jet printers, old binary CRT displays, laser xerography). Thus, given a color separation with 256 possible density levels or shades of color, a set of binary printer signals must be produced representing the contone effect. This process is referred to as halftoning.
Generally, in a halftoning operation, each pixel value in an array of contone pixels over a given area of the separation is compared to one of a set of preselected thresholds (the thresholds may be stored as a dither matrix and the repetitive pattern generated by this matrix is considered a halftone cell) as taught for example in U.S. Pat. No. 4,149,194 to Holladay. The effect of such an operation is that, for a given area of the image, some of the thresholds in the matrix will be exceeded, i.e., the image density level of the pixel value at that specific location is larger than the value stored in the dither matrix for that same location, while others are not. In the binary case, the pixels or cell elements for which the thresholds are exceeded might be printed, while the remaining elements are allowed to remain white or unprinted, dependent on the actual physical quantity described by the data. Since the human visual system tends to average out rapidly varying spatial patterns and perceives only a spatial average of the micro-variation in a printed area produced by a printer, the halftone process described above can be used to produce a close approximation to the desired color of that area in the contone input.
The dither matrix of threshold values is often referred to as a “screen”, and the process of generating the binary image from the contone image using the screen is called “screening”. Conventional digital halftones start as a number of isolated dots which grow bigger as more colorant is requested on the paper. These screens are referred to as clustered-dot screens. The fundamental spatial rate at which the dots in a clustered dot screen are repeated is commonly referred to as the screen's spatial frequency. The higher the screen spatial frequency, the finer and smoother appearing the image and also the greater is the capacity for the dots to represent fine detail in the image.
Dithering creates problems in color document reproduction where the repeating pattern of a screen through the image, when superposed over similar repeating patterns in multiple separations, can cause undesirable image artifacts, particularly in a printing system with less than ideal registration between separations. For example, it should be appreciated that dithering can cause “subject moiré,” wherein a period component in the image subject content beats, or interferes, with a screen frequency as well as color-to-color moiré, where the screens from different separations beat.
Dispersed dot screens are one alternative to conventional clustered dot screens. Dispersed dot screens are designed such that as the image density increases and more colorant (printed dots) is added, the added dots are not necessarily constrained to be adjacent to other each. Thus, instead of producing dots that grow in size with increased colorant on paper, dispersed dot methods grow in number and produce a well-dispersed pattern of isolated dots at spaced pixel locations. Dispersed dot screens generally provide higher spatial resolution than comparable clustered dot screens. Another advantage of dispersed dot screening over conventional cluster dot is the suppression of moiré.
One option for dispersed dot screening attempts to create a smooth dither pattern by locating dots within any specific intensity pattern such that they are spread as uniformly across the screen as possible. A recursive algorithm that produces such an optimal dither (the dots are as far apart as they can be at each level) is taught by R. E. Bayer, “An optimum method for two level rendition of continuous-tone pictures,” Proc. IEEE International Conf. on Communications, Conference Record, pp 26-11–26-15. More particularly, the Bayer screen has threshold values that are arranged such that when thresholded against increasing levels of density, the halftone dots are placed as far as possible from the other dots used to render lower density levels. However, images produced using such an “optimally smooth dither pattern” can be filled with objectionable patterns. Additionally, Bayer dispersed dots have many frequency components and, thus, the potential to beat with subject frequencies. A second option for implementing dispersed dot screens, uses a dither matrix wherein the dots are randomly scattered across the screen. However, use of a “truly random” screen such as would be representative of uniformly distributed and uncorrelated spectrum generally results in rather poor image quality.
Stochastic screening is an implementation of dispersed dot screening that combats the image artifacts associated with Bayer type screens and truly random screens. A stochastic screen contains dots with a random nature, and its halftone patterns can be less visible than structured halftone patterns produced by traditional clustered dot screens. In stochastic screening, the screen is neither truly random nor optimally smooth but rather is designed to produce patterns with pleasant noise characteristics. The pleasant noise characteristics are achieved by designing the screen so as to distribute the noise energy in the region of high spatial frequencies, where the human visual system has a significantly reduced sensitivity. Such uncorrelated, high frequency noise is often referred to as blue noise. Blue noise patterns have the desired aperiodic, uncorrelated structure of white noise without low frequency graininess. See, e.g., Digital Halftoning, R. A. Ulichney, MIT Press, Cambridge, Mass. 1987, (fifth printing, 1996).
Conventionally, stochastic screens have been designed such that the screen replicates a blue noise pattern as described above. A stochastic screen having a blue noise pattern traditionally is defined to mean a type of binary pattern produced after thresholding a gray-scale image (i.e., dot pattern) that has negligible low-frequency components and further possess the properties of isotropy and aperiodicity, which, when expressed in terms of the radially averaged power spectrum, has small or negligible low-frequency components, a transition region, and a high-frequency region which has an absence of stronger dominant spikes. That is, the dot pattern must have a collection of properties that must essentially include aperiodicity, isotropy (or low anisotropy), and lack of low-frequency graininess (i.e., dot patterns having a reduced number of dots per unit area). In this respect, a conventional stochastic screen has specific design goals as described above. That is, a stochastic screen is designed such that the power spectrum of the blue noise pattern has a small or negligible low frequency component and a high-frequency region which has an absence of dominant spikes and the resulting dot pattern (i.e., the image resulting from a screening process) is aperiodic, isotropic, and devoid of low-frequency graininess.
Information on designing and using a stochastic screen known as a blue noise mask can be found in a family of patents to Parker et al., including U.S. Pat. Nos. 5,111,310 and 5,477,305 which are herein incorporated by reference. The blue noise mask of Parker et al. meets the design goals for stochastic screens described above. More particularly, the family of patents to Parker et al. teach constructing an blue noise mask such that when thresholded at any level, the resulting dot profile is a locally aperiodic and isotropic binary pattern with small low-frequency components, which in the halftoning literature, is known as a blue noise pattern. Additionally, U.S. Pat. No. 5,673,121 to Wang, discloses a stochastic halftone screening method for designing an idealized stochastic screen and is herein incorporated by reference as it discloses a particular stochastic screen useful in implementing one or more embodiments of the invention, as will be more fully explained below. The idealized stochastic screen design method of Wang also provides classical stochastic screen representing blue noise.
Conventional stochastic screens with the design methods and goals of the prior art provide good image quality. Stochastic dots tend to be used in printing where either a very high frequency response is needed or you wish to avoid subject moire or color-to-color moire. They have found significant use in ink jet printing where the isolated dots are repeatable and thus local density is predictable and controllable. However, such conventional stochastic screens do not exploit the improvements in electrostatographic printing machines that have provided the ability to consistently and accurately produce small isolated dots using techniques such as high addressability, pulse width pulse position modulation (PWPM) or the like, and improvements in stability and uniformity of the marking processes. In such cases, and in many lithographic printing settings, it is possible to accept some degree of lowered stability (compared to clustered dots) to acquire the advantageous properties of high spatial resolution, and moire resistance. Thus, in accordance with the teachings below, there is described a method for halftoning an image using an anisotropic stochastic screen.
In one embodiment, the anisotropic stochastic screen generates a dot pattern that has an anisotropic power spectra while retaining the desirable characteristics of negligible low-frequency components and a high-frequency region which has an absence of stronger dominant spikes. Beneficially, the anisotropy achieved using a screen constructed of anamorphic pixels which can be obtained using conventional writing techniques such as high addressability, PWPM, or the like. One advantage of the present teachings is that the use of such anisotropic dots produces a screen with higher frequency content than conventional stochastic screens employing isotropic dots. Additionally, the teachings herein describe an anisotropic stochastic screen that achieves good dispersion of printed dots, results in an output image with good spatial frequency characteristics, and possess acceptable image quality.
In accordance with another embodiment disclosed herein, there is provided a method for halftoning an image by comparing a pixel of image data to a threshold level signal from a set of halftone threshold signals comprising a screen of anamorphic pixels, each threshold signal within the set of halftone threshold signals corresponding to an anamorphic pixel within the screen generating an output signal according to the comparison of the halftone threshold signal to the image data.
In accordance with another aspect of the teachings herein, there is provided a method of halftoning gray scale images by utilizing a pixel-by-pixel comparison of the image against an anisotropic stochastic screen, the anisotropic stochastic screen comprising a random non-deterministic, non-white noise function that, when thresholded, is designed to produce anisotropic dot patterns having a power spectrum characterized as having negligible low frequency components and a high-frequency region which has an absence of stronger dominant spikes.
In accordance with another aspect of the teachings herein, there is provided an apparatus for halftone image information, comprising a memory storing an anisotropic stochastic screen, the anisotropic stochastic screen including a set of halftone threshold level signals, each threshold signal corresponding to a unique location in a halftone cell and a comparator receiving a pixel of the gray scale image and one of the halftone threshold signals from the memory and producing an output signal according to the comparison of the halftone threshold signal to said image signal; wherein the anisotropic stochastic screen is comprised of a non-deterministic, non-white noise function which, when thresholded, produces an anisotropic dot pattern having a power spectrum characterized as having negligible low frequency components and a high-frequency region which has an absence of stronger dominant spikes.